a current injection folded switch mixer for direct conversion

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME

International Conference on Communication Systems (ICCS-2013) October 18-20, 2013 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 36

A Current Injection Folded-Switch Mixer for Direct Conversion Receiver

R Raja1, B Venkataramani2

1Research Scholar, 2Research Supervisor

Electronics and Communication Engineering, National Institute of Technology Tiruchirapalli, India

[email protected], [email protected]

ABSTRACT: In the literature, a Folded Switch Mixer (FSM) which uses self-bias current reuse technique at the transconductance stage is proposed and implemented in TSMC 90 nm CMOS process, for achieving high Conversion Gain (CG), good linearity and low Noise Figure (NF). In order to increase the CG and reduce the NF of FSM further, a Dynamic Current Injection FSM (CI-FSM) is proposed in this paper. For the purpose of comparison, FSM and CI-FSM are implemented in UMC 180 nm CMOS process and studied through simulation at 2.4GHz. From the study, it is found that the CG of CI-FSM is higher by 2dB and NF of CI-FSM approximately lower by 1.0dB compared to that of the FSM. But CI-FSM has maintain near same linearity (4dBm) compared to FSM (6dBm) at the cost of increase in area. KEYWORDS: Current Injection, Double-Balanced Mixer, Flicker noise, Folded Switch Mixer, Inductive Decoupling.

I. INTRODUCTION Advancement in wireless communication technologies enabled the development of wireless sensor networks (WSNs) and wireless body area networks (WBANs). WSNs and WBANs are employed in a wide variety of applications including health care and industrial automation due to their reliability, accuracy, flexibility, cost effectiveness and ease of deployment. Since majority of the nodes in the WSN are battery operated, the power consumed by the individual node should be reduced in order to enhance its life. Hence, designing the RF front-end blocks of WSN nodes with low power in the transceiver architecture becomes important. However, direct conversion architectures suffer from some drawbacks, the most important being DC-offset, local oscillator (LO) leakage and flicker noise. Generally BJT based transceivers used for GSM and WCDMA applications, and CMOS transceivers for wireless LANs, that use direct conversion, have been reported [1]. Zigbee standard is proposed for low power and low bit rate applications. Hence, this standard is assumed for the transceiver architecture in this paper. Radio frequency (RF) mixer is one of the important blocks in the RF front end which is used to either up-convert the signal to RF band or down-convert to base-band. Double-balanced mixer (DBM) topology is widely used for realizing down conversion mixers in direct

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conversion receiver architecture (DCR). Since DBM provides, high port-to-port isolation, high CG and lower dc-offset in DCR, this topology is preferred for transceivers using various wireless standards [2]. Hence, it is chosen for implementation in this paper. Gilbert cell topology may be used for realizing the DBM. However, it has a lower output swing due to stacking of transistors [3]-[5]. To overcome this, an ac-coupled folded switch mixer (FSM) with self-bias current reuse topology is proposed in [6]. This requires lower supply voltage and power consumption. But it has higher Conversion Gain (CG), Linearity (IIP3) and good port to port isolation. In order to increase the CG and linearity of the DBM using Gilbert Cell, current injection (CI) technique is proposed in [7]-[8]. This is at the cost of increase in the noise figure. To reduce the noise figure of DBM, CI with inductive decoupling is proposed in [10]-[11]. A dynamic current injection (DCI) scheme with tuned inductor is also proposed in [7] for reducing noise figure. In this paper, CI-FSM is proposed. This paper is organized as follows. An overview of dynamic current injection technique and ac-coupled Folded Switch Mixer are given in section II and section III respectively. Section IV presents the details of the proposed CI-FSM. Section V gives the implementation details and Simulation results of the Mixers. Section VI presents the conclusion. II. DYNAMIC CURRENT INJECTION TECHNIQUE

The double balanced Gilbert mixer (DBM) consists of trans-conductance (RF), Local Oscillator (LO) switch and Intermediate Frequency (IF) stages respectively. The flicker noise of the LO switch stage in Double Balanced Mixer (DBM) is the dominant source which degrades the Noise Figure (NF). For an output current I, the output noise current ‘io,n‘ due to flicker noise voltage ‘Vn’ can be expressed as [8],

i , = ∗∗

(1)

Where ‘T’ is the LO signal period and ‘S’ is the slope of the LO signal at the switching instant. From (1), it may be noted that the output noise current can be reduced either by increasing ‘S’ or by decreasing the ‘Vn’. Since, decreasing ‘Vn’ requires larger device size, ‘S’ has to be increased to reduce ‘io,n’. This in turn reduces the noise period‘t’ given by,

t = Vn(t)/S (2)

To reduce‘t’, the dynamic current injection (DCI) technique is proposed for DBM in [8] and the circuit for DCI is shown in fig. 1. In Fig. 1, the current source ID and the switches achieve the dynamic current injection. The voltages at the nodes A and B of fig.1 turn the switches ON and OFF. Since the LO amplitude is large, the voltages at nodes A and B are as shown in Fig. 2. The switches turn ON when the voltage reaches a minimum level or zero crossing point and thus, the current Is1 and Is2 are injected to nodes A and B. This effectively reduces height of the noise pulses at the output to zero as shown in Fig. 2 and eliminates the flicker noise. When no current is injected, the mixer operates normally. In the fixed current injection scheme [8] shown in fig. 3, current IF is always injected to the common source node of LO stage. But in DCI scheme, when the LO differential pair is balanced, the current is injected dynamically and DCI switches do not add noise at the output, since they are ON only at the switching instants.


International Conference on Communication Systems (ICCS-2013) October 18-20, 2013 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India 8

Fig. 1: Dynamic current injection technique for DBM

Fig. 2 Local oscillator common source node voltage waveform of DBM (i.e @ node A and B)

III. AC COUPLED FOLDED SWITCH MIXER The conversion gain and linearity of double balanced mixer are enhanced in [6] by modifying the RF transconductance stage with a self-bias current reuse technique. This mixer is termed as ac-coupled folded switch mixer (FSM) and is shown in fig. 4. In fig. 4 M1, M2 form the RF transconductance stage. M3, M4 form the self-bias current reuse circuit. The transistors M5–M8 form the pMOS Local Oscillator (LO) folded switch cores which are used to reduce the contribution of flicker noise at the output. In this FSM, two ac-coupling capacitors, two LC tanks and two IF loads (Rload) are used. To prevent the signal leakage through the power supply, the values of ‘Lg’ and ‘Cg’ in fig.4 are chosen such that they resonate at the required operating frequency. From [6], it may be noted that the conversion gain (CG), IIP3 and NF of the FSM are given by CG = 20log 2 g + g R (3)

IF output

LO+ LO+ LO

--

RF+- RF--

Cp Cp ID

IS1 IS2

A B



Fig. 3 Current injection technique for DBM

IIP3= 10log (A2IIP3/1mW*2*50) (4)

NF = 10 log

⎝

⎜⎛

1 +V

,+ V , h

V ,

⎠

⎟⎞

(5)

Where g , g are the transconductance of nMOS and pMOS transistors in the self-bias

current reuse circuit, A is Peak-Peak of LO signal, ′푉,′ is flicker noise and ‘푉 ,

′,

′푉 , ′ are thermal noise and source noise of the LO switch pair respectively. IV. DYNAMIC CURRENT INJECTION FOLDED SWITCH MIXER Dynamic current injection scheme using a pMOS Switch with tuned inductor ‘Ls’ is proposed in the literature [10]-[11] for DBM to avoid the flicker noise during the ON/OFF transition of LO switch. This scheme is adapted for the folded switch mixer in this paper and is termed as current injection FSM (CI-FSM). The circuit diagram of the CI-FSM is shown in Fig. 5. In CI-FSM, the dynamic current injection is realized using a current source M11 and two cross coupled pMOS transistors M9 and M10, whose gates are connected to node ‘X’ and ‘Y’ through tuned inductors. The non linear capacitances of the pMOS device produce harmonics and form leakage paths for injection current. This may be minimized by using a series tuned inductors ‘Ls1’ and ‘Ls2’ to resonate with parasitic capacitances at both pMOS switch pair and LO switch source at the second harmonic of the LO frequency (2f0).

IF output

LO+ LO+ LO

--

RF+ RF--

Cp Cp IF

IF+iRF

A B



Fig. 4 The ac-coupled folded switch mixer

Fig. 5 Schematic diagram of proposed CI-FSM

The proposed CI-FSM circuit effectively monitors the zero-crossing at the LO Common source node and efficiently injects the small aperture current in the transconductance stage to further improve the transconductance without affecting the bias current of switching stage. Thus the flicker noise is reduced at the output of the FSM. Sizes of pMOS switches are chosen to avoid the parasitic effect in the source terminal of LO switch pair.



V. SIMULATION RESULTS The FSM and the proposed CI-FSM are implemented in 0.18-m CMOS process and simulated using the Cadence SpectreRF Simulator. For the purpose of comparison, FSM and CI-FSM are designed for same bias current of 3.5mA and supply voltage of 1.8V. The inductor values ‘Ls1’, ‘Ls2’ in CI-FSM are 11.6nH and layout area for this inductor in 180nm technology is 453x453µm2. The mixer is designed to operate between 2.4 – 2.48GHz with a local oscillator (LO) power of 15dBm and IF of 5MHz. For different RF and LO frequencies, the performance metrics such as Conversion gain (CG), Linearity (IIP3), Noise figure (NF) and port-port isolation etc. are computed for FSM, and CI-FSM are given in the table 1. Fig. 6 shows the simulation results of LO power Vs Conversion gain for both FSM and CI-FSM. The CG of FSM and CI-FSM computed at 2.4 GHz is found to be 13.2dB to 15.34dB respectively. The increase in CG of CI-FSM is due to increase in the load resistance (Rload). The increase in ‘Rload’ is due to same bias current (Ibias) without disturbing the operating voltages of the LO switching pair. Fig. 7 shows the simulated noise (NF) of mixer for different IF up to 5MHz. The noise figure (NF) of FSM and CI-FSM are found to be 9.06dB and 8.18dB respectively. From (5), it may be observed that the decrease in Von,(1/f) of the LO switch core leads to decrease in noise figure(NF). This is achieved by DCI technique. Fig. 8 gives the plot of RF input power Vs IF output power. From this figure, it can be noted that the third-order inter modulation input intercept point (IIP3) of the FSM and CI-FSM are 4.08 dB and 6.19dB respectively. From the table 1, it can be noted that both the FSM and CI-FSM provides the better RF-to-LO isolation, LO-to-RF isolation, RF-to-IF isolation, and LO-to-IF isolation. For the purpose of comparison, the figure of merit (FOM) is given by [6]-[7] FOM = 10log {((10 CG/20 *10 (IIP3-10) /20 ) / (10 NF/10 * Pdc ))} (6) is computed for different FSM and CI_FSM and is given in table. 1. Here the ‘Pdc’ is the power consumption. The FOM of CI-FSM is better and hence it may be preferred at the cost of increase in area.

Fig. 6 Local oscillator power Vs Conversion gain

…. FSM



Fig. 7 Variations of noise figure with IF

Table 1: Performance Summary of the FSM and CI-FSM

Parameters Performance Metrics

FSM CI-FSM 퐕퐃퐃(V) 1.8 1.8 퐈퐃(mA) 3.5 3.5 IF frequency (Hz) 5M 5M Conversion Gain (dB) 13.2 15.34 NF (dB) 9.07 8.19 IIP3 (dBm) 6.19 4.08 IP1db (dBm) -6.07 -7.7 RF-to-LO isolation (dB) -53 -53 LO-to-RF isolation (dB) -48 -46 RF-to-IF isolation (dB) -108 -98 LO-to-IF isolation (dB) -97 -95 Power Diss. (mW) 6.3 6.3 FOM 17.3 18.6

Fig. 8 RF-input power Vs IF-output power

…. FSM

----CI-FSM



V. CONCLUSION The ac-coupled folded switch mixer proposed in the literature and the CI-FSM proposed in this paper are implemented in the UMC 180nm CMOS process and studied through simulation. From this study, it is found that the CI-FSM has higher CG and lower NF. This is achieved at the cost of minor degradation in IIP3 and increase in area. The CI-FSM has a better Figure of Merit compared to FSM. The proposed FSM works satisfactorily at 2.4 GHz band and is suitable for direct conversion receiver. REFERENCES [1] Danilo Manstretta, Rinaldo Castello, and Francesco Svelto, “Low 1/f Noise CMOS Active

Mixers for Direct Conversion”, IEEE Transactions On Circuits And Systems—Ii: Analog And Digital Signal Processing, Vol. 48, No. 9, September 2001.

[2] V. Vidojkovic et al.,”Mixer topology selection for a 1.8-2.5 GHz multi-standard front-end in 0.18µm CMOS,” in Proc. IEEE Int. Symp. Circuits and Systems (ISCAS), vol. 2, 2003, pp. 300-303.

[3] Behzad Razavi, “RF Microelectronics”, Prentice Hall Communications Engineering and Emerging Technology Series G

[4] Thomas Lee, “The Design of CMOS Radio Frequency Integrated Circuits”, Cambridge University Press.

[5] Behzad Razavi, “Design of Analog CMOS Integrated Circuits," 1st Ed., McGraw Hill, 2001 [6] Hwann-Kaeo Chiou, Member, IEEE, Kuei-Cheng Lin, Wei-Hsien Chen, and Ying-Zong Juang

“A 1-V 5-GHz Self-Bias Folded-Switch Mixer in 90-nm CMOS for WLAN Receiver” in IEEE Tractions on Circuits and Systems—I: regular papers, vol. 59, no. 6, June 2012

[7] Jehyung Yoon, Student Member, IEEE, Huijung Kim, Changjoon Park, Student Member, IEEE, Jinho Yang, Hyejeong Song, Sekyeong Lee, and Bumman Kim, Fellow, IEEE “A New RF CMOS Gilbert Mixer With Improved Noise Figure and Linearity” IEEE Transactions on Microwave Theory and Techniques, vol. 56, no. 3, March 2008.

[8] H. Darabi, Senior Member, IEEE, and J. Chiu, “A Noise Cancellation Technique in Active RF-CMOS Mixers” IEEE Journal of solid state circuits, Vol. 40, No. 12, December 2005.

[9] Vojkan Vidojkovic, Johan van der Tang, Member, IEEE, Arjan Leeuwenburgh, and Arthur H.M. Van Roermund, Senior Member, IEEE“A Low-Voltage Folded-Switching Mixer in 0.18-um CMOS” IEEE Journal of Solid-State Circuits, Vol. 40, no. 6, June 2005.

[10] S. Douss, Farid Touati, Mourad Loulou“A 3.1-4.8GHz CMOS Mixer Design Using Current Bleeding Technique For UWB MB-OFDM Receivers” 2007 IEEE International Conference on Signal Processing and Communications (ICSPC 2007), 24-27 November 2007.

[11] Amir Hossein Masnadi Shirazi Nejad,"On The Design Of Low-Voltage Power-Efficient Cmos Active Down- Conversion Mixers", The University Of British Columbia, Master's Thesis,

April-13. BIOGRAPHY

B Venkataramani received his B.E. degree in Electronics and Communication Engineering from Regional Engineering College, Tiruchirappalli in 1979 and M.Tech. and Ph.D. degrees in Electrical Engineering from Indian Institute of Technology, Kanpur in 1984 and 1996, respectively. He worked as Deputy Engineer in Bharat Electronics Limited, Bangalore, India, and as a research Engineer in Indian Institute of Technology, Kanpur, each for approximately 3years. Since 1987 he has been



faculty member of National Institute of Technology (formerly Regional Engineering College) Tiruchirappalli. Presently, he is working as Professor in the Department of Electronics and Communication Engineering in National Institute of Technology Tiruchirappalli. He has published two books and numerous journal papers and international conferences. His recent research interests include Software Defined Radio; interconnect modeling and mixed-signal Integrated circuits design.

R Raja received his B.E degree in Electronics and Communication Engineering from Anna University, Chennai in 2005 and M.E from Anna University, Coimbatore, India in 2010. He worked as a VLSI Design Engineer in India for a period of 3 years and worked as a Faculty in Electronics and Communication Engineering department for a period of one year. Presently, he is pursuing his Ph.D in Department of Electronics and Communication Engineering, National Institute of Technology Tiruchirappalli. His research interest includes

Analog/RF IC design and design of Analog frontend modules for Wireless Communication standards such as Zigbee, WLANs, PANs, CDMA/GSM based transceivers.

a current injection folded switch mixer for direct conversion

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